At last year’s EGU meeting several late-breaking sessions covered Iceland’s Eyjafjallajökull volcano, which was still erupting. This year there was plenty of time to schedule sessions well in advance and, when it came to ash, more of on emphasis on its effects on the ground rather than the consequences of airborne ash for aircraft safety.
As a result of the eruption, farmland 15 km south of the volcano was coated with an ash layer more than 10 cm deep. Pierre Delmelle from the University of York, UK, believes he and his colleagues are the first to study the physical effects of ash on soil. There’s usually more attention paid to chemical effects but Delmelle reckons that for the Icelandic volcano, the physical effects may be just as important.
Delmelle found that, when fine ash was ploughed into the soil, its permeability to water decreased, probably because of a change in pore size distribution. In the absence of ash, the soil exhibited a hydraulic conductivity – a measure of the ease with which water can flow through it – of around 0.9 mm/second. The figure for soil containing fine ash that incorporated sulphates and fluorides, however, was just 0.25 mm/s.
Water flow through soil is important for agriculture as it affects the distribution of nutrients, and soil moisture is a key factor for healthy plant growth. While such a reduction in permeability is unlikely to be hugely detrimental to the well-draining soils in Iceland, in volcanic areas such as Indonesia, or following a super-volcano, it could lead to water-logging.
With regards to chemical effects, the main concern for soil is the presence of fluoride in the ash, which can harm plants, livestock and people when it gets into the food chain.
Delmelle found that ash from the second phase of the volcano’s eruption – from 18th April until the end of May – contained eight times more soluble fluoride than ash emitted in the first phase, between 14th and 18th April. This initial ash had less than 200 mg of soluble fluoride per kg.
Delmelle believes that the steam present during the first phase of the eruption scavenged fluoride from volcanic gases. In the second, water-free, phase the ash was able to take up this fluoride instead.
However, it seems that ash from the two phases contained similar levels of acid-soluble fluoride, particularly fluorapatite. Since acid conditions occur in the guts of cows, sheep and humans, this is potentially an issue of concern, although Delmelle does not believe that it will cause diseases such as fluorosis in Iceland.
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